1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by using an electrophotographic system, and a process cartridge used in such an image forming apparatus.
The image forming apparatus may be, for example, an electrophotographic copying machine, an electrophotographic printer (for example, LED printer, laser beam printer or the like), an electrophotographic facsimile, an electrophotographic word processor or the like.
2. Related Background Art
In image forming apparatuses using an electrophotographic technique, a uniformly charged image bearing member (photosensitive drum) is selectively exposed to form a latent image on the image bearing member, the latent image is visualized by a developing agent (toner) as a toner image, and the toner image is transferred onto a recording medium. The transferred image is fixed to the recording medium. In this way, image recording is effected. After the transferring, residual toner remaining on the photosensitive drum is removed by a cleaning means, and, then, the photosensitive drum is uniformly charged by a charge means, thereby preparing for next image forming process.
In such image forming apparatuses, replenishment of developing agent and maintenance such as adjustment, cleaning and replacement of various process means (charge means, developing means, cleaning means and the like) are required. In order to facilitate the maintenance, there has been proposed a process cartridge in which the photosensitive drum, and at least one of a charge means, a developing means and a cleaning means are collectively housed in a cartridge frame.
As the cleaning means, a cleaning apparatus in which a cleaning blade, made of elastic material such as urethane rubber, is urged against a photosensitive drum to scrape residual toner (waste toner) from the photosensitive drum and the waste toner is collected into a cleaning frame has been put to practical use. In order to realize effective cleaning, it is required that the cleaning blade is surely held at a predetermined position to urge the cleaning blade against the surface of the photosensitive drum with uniform pressure. To this end, the elastic cleaning blade is attached to a cleaning blade holding member having high rigidity and the holding member is attached to the cleaning frame. As the cleaning blade holding member, a blade holding metal plate having high rigidity obtained by a bending technique has been widely used.
In order to prevent the waste toner scraped from the photosensitive drum from leaking out of the frame, an elastic dip sheet for closing a gap between the cleaning frame and the photosensitive drum is disposed at an upstream side of the cleaning blade in a rotational direction of the photosensitive drum. In order to pass the residual toner on the photosensitive drum through the dip sheet and to scrape the residual toner by the cleaning blade, it is required that the dip sheet is surely urged against the photosensitive drum with pressure, which does not scrape the residual toner.
As shown in FIG. 8 (showing a conventional example) and FIG. 9 (showing the present invention, but, conveniently used for explanation of the conventional example), since a cleaning frame 52 has a box shape one face (facing to a developing device 40) of which is opened, it is difficult to ensure torsional rigidity of the frame itself. Since a blade holding metal plate 51b is fastened to the cleaning frame 52 by small screws 55 at both ends thereof, torsional rigidity at both ends of a process cartridge is enhanced, but, torsional rigidity of a central portion of the process cartridge depends upon the torsional rigidity of the cleaning frame 52 itself, the torsional rigidity of the central portion is weak, with the result that a reduction in cost obtained by reducing a thickness of the cleaning frame 52 cannot be achieved.
Particularly, as is in the illustrated example, in the process cartridge 2 in which guide bosses (means for positioning and angle-determining the cartridge in a mounted condition) 52e are formed on both longitudinal ends of the cleaning frame 52 and a grip 52f is formed on a central portion of the cleaning frame 52, when the process cartridge 2 is mounted to an image forming apparatus 1 (refer to FIG. 1), as shown in FIGS. 10 and 11, the guide bosses 52e are fitted into guide grooves 1a formed in the image forming apparatus 1. Thus, since a force direction toward a direction shown by the arrow F in FIG. 11 acts on the grip 52f under the condition that the both ends of the cleaning frame 52 are fixed by the guide bosses 52e, torsional stress for separating a dip sheet 54 from a photosensitive drum 20 (in a direction shown by the arrow R) is generated on the central portion of the cleaning frame 52.
Consequently, if the torsional strength of the cleaning frame 52 is insufficient, as shown in FIG. 12A, a free end 54a of the dip sheet 54 is separated from the photosensitive drum 20, with the result that toner stored in the cleaning frame 52 may be flown out. Particularly when the dip sheet 54 is formed from material having poor expansion/contraction, since the deformation generated at an attachment surface 54b of the sheet cannot be absorbed, as shown in FIG. 12B, undulation is generated at the free end 54a to easily create gaps between the dip sheet and the photosensitive drum 20. In the past, in order to prevent flow-out of the toner, it has been required that the thickness of the cleaning frame 52 is increased or reinforcing ribs (not shown) are provided.